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Could a quadrotor use a main gas powered rotor with small electric rotors for attitude control?
Here is my idea of heavy lifting quadcopter. (Although it is not a quad copter anymore because of the center rotor)
Have a big coaxial fixed pitch rotor at the center. The rotor is gasoline fueled.
Have the other rotors at extended length with electric motors to control pitch yaw and roll.
The center coaxial rotor will do the heavy lifting.
Can it work? What are the difficulties?

Building the transmission and concentric shaft monkeyworks...  Coaxial rotors are hard.  Another option might be intermeshing rotors like the Kaman K-max, noting its lack of swashplate... The quadrotors would let you dispense with even the K-max's wingtip ailerons.

the whole idea is to avoid variable pitch blade.

Should you get it to work, it wouldn't be a quadcopter!

Early versions of Sikorsky's VS-300 helicopter prototype used something similar to what you're describing, as his first attempts had three rotors on the tail boom, one vertical rotor for anti-torque and yaw, and two horizontal rotors for pitch and roll.

Of course, if you're already using large main rotors, you might find, as Sikorsky found, that cyclic pitch control works even better and eliminates the need for the two horizontal rotors.

Yes that can work - with some difficulties and limitations. Autogiro's don't have variable blade pitch and they can fly, but they also don't have the reaction yaw issue due to driving the main rotor.

One big fixed rotor doing the heavy lifting - let's start with the device in the hover and trimmed out by the four electrical trim rotors. Then it needs to climb, since the main rotor blades are fixed pitch the rotor needs to speed up => increased yawing moment => the device starts to spin. You would need something like a tail rotor to compensate for only yaw. If the quad electric rotors are dimensioned for trimming and tilting only, reducing their rpm would not compensate for the increased torque of the main rotor.
Now we want to propel it forwards. Since the quadcopter rotors are pointing downwards the whole craft needs to be tilted, creating gyroscopic bending moments on the rotor shaft and on the blades as well if they don't have flapping hinges. Control-wise that can be accommodated of course, just apply a precessed torque on the craft so that it tilts forwards. But it would be simpler if you would have a pusher propeller installed so you don't need to tilt.
Then the craft starts to fly forwards. You must have flapping hinges now, otherwise the craft will roll over; lead-lag hinges for preventing fatigue failures. The rotor power will decrease with forward speed at first due to translational lift, decreasing the reaction yaw. We need that tail rotor again, and/or a stabilising vertical fin.

To me it sounds like this design is halfway a quadcopter and an autogiro, combining mostly the drawbacks of each design. The quadcopter has simplicity of control as its main redeeming feature, this vanishes when you add a big, engine driven rotor. As to the drawbacks of the autogiro....ehm. I cannot think of one.
Image source
The autogiro's development history is a point in case:  De la Cierva's first designs used a conventional elevator and ailerons on stub wings for pitch and roll control, these were not very effective at low speeds so he made the rotor being able to tilt on a gimbal with respect to the shaft. Later as the rotors got bigger, the forces to tilt them became too high and the mechanism for cyclic blade pitch was developed. All of this in the 1920s/1930s!
So while your design seems possible, I cannot see an advantage over an autogiro with a tilt-able rotor. One big horizontal rotor for lift, one small vertical rotor for thrust.

It's true that present-day autogyros have simple rotor heads with no feathering control, but that wasn't the case of the more advanced autogyros of the 1930s and 40s., where the blade pitch could be controlled by the pilot. Anyway, even in the simple Bensen-type autogyros that can be seen in flight today, the flapping action does vary the AoA of the turning blades, having an effect that is equivalent to periodical feathering...

remember what i propose is central coaxial counterrotating rotor. so there would be no yawing motion for change in rotation speed.


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Do turboprop and piston engines perform differently at higher altitudes?
When flying at higher altitudes, does it matter if you have a turboprop vs a piston engine driving a propeller?

In general, a turbine engine will produce more power than a piston engine at higher altitudes. A turbocharged piston engine, with compressed air intake, will lose less power at altitude than an normally aspirated piston engine. A turbine engine will lose even less.
As a rough example, a normally aspirated Cirrus SR-22 has a service ceiling of 17,500 ft. A turbocharged SR-22 has a ceiling of 25,000.
A turbocharged piston Piper Malibu Mirage has a service ceiling of 25,000 ft., while the turbine powered Malibu Meridian has a ceiling of 30,000 ft.
The turboprop Pilatus PC12 and Socata TBM-850 have service ceilings of 30,000 ft. or above.

Might be worth comparing a pressurized piston single like a Cessna P210 or Piper Malibu - they're more directly comparable to the pressurized turbine singles you mention! Both have a service ceiling of 25,000 ft, but as with the turbo Cirrus I think that's a certification thing rather than performance.

Good point. I added Malibu Mirage and Meridian.

For most of these aircraft, the altitudes you are actually maximum operating altitudes for certification purposes, not (necessarily) the same as the service ceiling (100fpm climb rate). However, what you said generally is pretty much correct.

Good point. Now that you mention it, I seem to remember the max operating altitudes on the Pilatus and Socata are set because of pressurization limits.

I don't thing this statement is correct: "A turbocharged piston engine, with compressed air intake, will lose less power at altitude than an normally aspirated piston engine. A turbine engine will lose even less."  Depending upon the turbocharging system the piston system may still be producing the same power at ground level and at high altitude.  A turbine will lose power as the plane climbs, but has a lot more power to start with.

Of course it does!
One simple way of looking at this is that piston-powered (well cylinders) are constant-volume engines (ie. 1200cc). As you go higher the air becomes less dense so there is less air mass (and fuel) getting into the engines. If the density becomes half what it was at sea level, you now have in effect a 600cc equivalent engine, etc. Power will be less and less.
A turboprop is a constant pressure engine, so as you climb, you kinda need to go faster and faster, but basically you still burn the same amount of air/fuel. You will lose power as you climb in a turboprop,  but that's mostly cause the propeller loses efficiency at the higher speeds you fly higher up. The turbine itself is still as efficient as on the ground.

The turbine has it's limits too. A jet engine will at FL350 produce less than half of what it can do at sea level. But the power does indeed decline much more slowly.

+1 for constant volume vs constant pressure. The efficiency mystery makes sense now (:

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How does the MD 900 accomplish auto rotation with the NOTAR system?
The MD-900 is a helicopter which seems to be quite popular with law enforcement agencies.

As you can see, instead of an anti-torque tail rotor, a fan exhaust is directed out slots in the tail boom. I was wondering if this works in regards to auto rotation, should the aircraft lose its engines.

During an auto-rotation, you don't need torque compensation at all. Those vertical stabilizers are more than enough. Autogyros fly in auto-rotation all the time, and they don't need any torque compensating device, because in this type of aircraft the rotor is not powered by an onboard engine, but rotates freely, powered by the relative wind. In the absence of a torque applied by the engine to the rotor, there is no opposite reaction; hence, no compensation is necessary... A helicopter descending in autorotation is exactly like an autogyro gliding with the engine off...

i think your answer is true. but maybe youll need to add some details to answer. i think you should add these, why you dont need torque compensation when engine is not running. why you normally need it at a helicopter. whats the difference between helis and gyros and what are their design pricibles to need or not to need torqu compensating. why auto gyros dont need torque compensation.

You're suggesting it could autorotate on a straight glidepath.  But unless those vertical stabilizers happen to have rudders then there would be no means to effect a controlled turn.

According to https://en.wikipedia.org/wiki/NOTAR the NOTAR fan is driven by the main rotor transmission. This will ensure maneuverability during autorotation.

This, plus the fact that in autorotation the rotor is not being driven by the engine, so there is far less torque acting on the fuselage. That's not the reason it works, but it's an alleviating factor

The flow through the rotor plane in a helicopter in autorotation is upwards, hence there is no rotor downwash blowing on the boom, and the NOTAR system probably won't work well –if at all– under those conditions...

@xxavier good point. Please feel free to edit / add. It might be that the rotation of the exit can be adjusted to compensate for the flow change.

In the highly respected book 'The Art of the Helicopter', by John Watkinson, chapter 5-11 (NOTAR) the following can be read: 'In the case of autorotation, the boom will be in upwash and the boundary layer control is no longer effective, but nor is it needed. The fan is driven from the main rotor and yaw control is by rotating the tail sleeve as usual'. In my opinion, this closes the discussion...

It's close to truth but not true. Helicopters in autorotation stil produce torque due to rotor spinning, which is beared to the fuselage and transmission also working producing antitorque force which is required not only to countertorque but to steering turns of helicopter to choose landing spot, avoid obstacles etc Maybe vertical stabilisers offers some help but notice that emergency landing procedure entry may be coerced in hover state and also ended in hover when stabilisers not working of course.
Gyrocopters :
1 gearbox never attached when you fly - only friction of rotor.
2 Gyrocopters virtually never hover - take off, flying, landing - stabilisers always works.

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Are NATO fly-by-wire fighters designed to feel like F-14 Tomcats?
Related question:  Do fly-by-wire flight controls in airliners provide artificial feel?

Icefire, by Judith and Garfield Reeves-Stevens, contains the following passage:

When the engineers develop a new plane, they like to give the pilots something familiar to work with, so they adjust the computer controls to feel like a plane the pilots already fly and like.  The Tomcat's one of those jets.  Solid, dependable, every test pilot's flown them.  So every new fly-by-wire plane feels a lot like a Tomcat.

Now, let's face it, this is a book featuring the main character stealing, and making off with, a Lockheed SR-72, so I'm not expecting everything to be a model of technical accuracy, but there could be a rock of truth in the middle of this snowball of fiction.
Are NATO fly-by-wire fighters designed to feel like F-14 Tomcats?

It seems that the question itself is a tautology.  give the pilots something familiar - since most NATO nations do not operate the F14 nor have their pilots flown one, then why chose it as the "model"?

I'd guess that the common model is not the F-14 but the range of movements and resistances that facilitate sustained fine-control in average 20-year old males (and now people of any sex). It may be the F-14 gets close to this but that correlation is not causative for later designs.

If you can actually steal an SR-72, manage to takeoff, and know how to fly the thing, I'd think making off with it would be pretty easy, at least until you run out of fuel. I mean, what are they going to do when you blast away at Mach 6? An F-22 would look like it's sitting still by comparison. The missiles fired by an F-22 would look like they're sitting still by comparison. However, things become more problematic when you need to refuel.

Probably not.
One manufacturer of "Active Control" joysticks, as used in F-35 training simulators says

Because the active control stick is controlled by a computer almost any characteristic can be programmed. Typical and useful characteristics include:

Multi modal feel characteristics - the ability to completely change the stick feel when moving from one flight condition to another, for example, in a vertical take-off and landing aircraft from wing borne to jet borne flight.
Variable stiffness characteristics - the control forces can be changed forcing the pilot out of dangerous manoeuvres.
Variable range of motion - the available motion range can be changed forcing the pilot out of dangerous manoeuvres.
Soft stops - localised increases in the force characteristics can be programmed dramatically into the stick characteristic to warn the pilot of the onset of flight restrictions.
Power management cues - in helicopters a major maintenance expense is incurred if the pilot applies too much torque to the rotor. The stick force can be made to reflect the rotor torque through the flight envelope reducing pilot workload in monitoring the torque and operating costs.
Stick shake functions - to mimic buffet ad issue warnings to the pilot.
The stick can also be made to reproduce the mechanical characteristics of a more conventional stick such as breakout forces at the datum.

I'm unfamiliar with F-14 control stick behaviour but I imagine it has none of the first five characteristics though it might have the last two of these.
I don't know if the F-35, for example makes use of these characteristics but it seems likely.

Most decidedly not, at least not for the first fly-by-wire fighter in the NATO arsenal, the F-16. Its stick, in initial block versions, did not move at all, instead using piezoelectric load cells to detect stick pressure. This caused some issues with novice Viper drivers overreacting to the stiffness of the controls compared to anything they had flown and overrotating the jet on takeoff, smacking the turkey feathers on the ground. Later blocks put a little play in the stick.
As far as the Tomcat being an easy plane to fly, Top Gun kind of over-romanticized it. The TF30 engine initially put into these aircraft was underpowered and terribly unreliable (one thing the movie did get right is that the engines were prone to compressor stalls when encountering disturbed air such as jetwash or even their own missile launch trails). The aircraft itself, while a big improvement in dogfighting over the F-4, was still big and heavy, the result of requirements that it be a long-range supersonic interceptor in an era without supercruise or the relatively light AMRAAM missile; the aircraft instead had to be able to haul a full load of AIM-54 Phoenix missiles intended for the ill-fated naval variant of the F-111, along with the large, complex radar system to lock on and fire them, and a boatload of fuel to get all that out to patrol range and back.
The Navy's initial mixed results with the F-14A, along with the extremely high costs of the aircraft ($32 million each at the height of production) and the lack of ground attack capabilities, were mirrored in the USAF with its F-15 program; the looming retirement of the F-4 was about to leave both branches without a fighter capable of the Phantom's multi-mission capability, and both the F-14 and F-15 were big expensive interceptor-style pure air-to-air fighters, which some Air Force brass believed was proof that nothing had been learned from Vietnam. The USAF commissioned the Light Weight Fighter competition, which produced the F-16, a light, relatively inexpensive multirole fighter that has become one of the most successful land-based fighter jets in history. The Navy preferred some traits in the USAF's losing aircraft, the YF-17, such as the twin-engine design for durability, and it would go on to develop this design into one of the most successful naval aircraft of all time, the F/A-18 Hornet and Super Hornet.
The Tomcat eventually lived up to its promises with some engine and avionics upgrades (including a hybrid-FBW system and a LANTIRN pod to add air to ground smart bomb capabilities) but the aircraft was by no means a perfect plane or a dream to fly.

Really nice answer. Would be spectacular with some references edited in.

The post above comments on the F-16 originally having an immobile control stick, and that it had been modified in later versions to be more similar to a video game. As a professional video game developer who has actually been in a late-model F-16 simulator, I can tell you that it feels nothing like a video game stick, and it feels a bit like pushing on an immobile metal rod that has been covered in a slightly spongy surface. It doesn't actually move in any direction, although the surface does have a bit of give, so it doesn't feel totally immobile. Despite the fact that I had never used one before, it was still relatively intuitive to use, and I didn't find it difficult, but it is dramatically different than a joystick. I haven't been in any other jet cockpits, so I don't know for sure, but I would assume they would all have to be dramatically stiffer than anything used in civilian life, if for no reason other than to avoid having it move during acrobatic maneuvers.


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Does the FAA have a best practice for highlighting memory items on a checklist?
voretaq7 mentioned the following two months ago in an answer:

The process of designing a good checklist is an interesting Human Factors problem - there's a lot of literature on it but it's way out of scope for this site. The usual solution to badly designed checklists is that pilots get frustrated and write our own.

Funny he mentioned that, because I'm frustrated with a badly designed checklist and I'm writing my own.
Are there FAA-specific best practices for creating checklists in general and highlighting memory items in this specific case?
By this I'm referring to underlining, red-boxing, font changes, color changes, and so on.

uhm, I feel this is borderline. might be better off in UX.SE

I've edited to make it less UX-centric but migration would be fine too.

@Federico, I fear ux.se is a bit too computer user interfaces-centred while checklists are mostly aviation-specific things. That said, it probably still makes sense to also try asking there.

The short answer is to use color in some way.
I couldn't find anything very detailed on this: there's plenty of general checklist information out there from the FAA, NASA, universities and so on, but nothing that goes into specific details on how to use colors, fonts or the other things that you mentioned.
There is an FAA report called The Use and Design of Flightcrew Checklists and Manuals that was part of the response to the Northwest 255 crash. Unfortunately it's from 1991 so some content is already dated, and it's aimed at part 121 and 135 operators with multiple crew members, but it's an interesting report anyway.
Their main recommendation on memory items was simply to reduce them as much as possible because memory is very unreliable compared to checklists, but again that's in the context of multi-member crews. As for design, layout, formatting etc., they did make some specific recommendations (see section 4.1 and Appendix A) and the most applicable one to your question seems to be the use of color-coding. They strongly recommended the use of color as a general method, e.g. headings in yellow for "abnormal" items and in red for "emergency" items.
Interestingly, the report says that at that time (1991) airlines resisted using color because of the costs of printing, but today it seems to be the generally accepted way to highlight critical checklists and items. However, each checklist provider has their own approach to coloring headers, text, blocks or even entire pages.
I don't think there's any such thing as a standard 'style guide' for checklists, so if you're making your own then you can obviously do whatever you want that works for you. But the 1991 report noted that most checklist problems were caused by missing or inaccurate items, so make sure you get the content right before you break out your paintbrushes :-)

Good answer. One thing; most theories of graphic design assume the results will be viewed under a sufficiently bright white light. That's not always the case in an aircraft; your map light might be dim, or even a different color (red is common for military use to preserve night vision). That can cause normally high-visibility design elements to dramatically lose contrast; for instance, under red light, red text or lines on white paper disappear, and blue and green.become indistinguishable from black. It might never be a problem, or it can cause entire checklist steps to be indecipherable.

@KeithS The 1991 report mentioned that a study found that black type on "bright lemon yellow" paper gives the best readability in varying cockpit lighting conditions. Of course that was specifically about checklists, not charts or anything else. And yellow "abnormal" text on yellow paper wouldn't work so well :-)

There is no guidance as far as I am aware of.
Most good pilots make their own checklists for smaller aircraft. Generally speaking different pilots like different levels of detail and size formats for the checklist. So, you might have one pilot with an 8.5" x 11" monster checklist with a lot of detail and another guy who is using an index card strapped to his forearm.
I would keep the color and symbology to a minimum. You cannot see color at night. Also, other people may need to use your checklist or may look at it (like a flight examiner). You do not want to have cryptic elements on a checklist that could be misinterpreted.
There is an advantage to simplicity.

"Checklists" are discouraged, "Worklists", "DoLists" and "Flow" are the new thing. They rely on some pilot memory and familiarization. How the pilot remembers is up to his own style, but usually a top to bottom, left to right "Flow" (explained later) is used.
The FAA realizes that counting on some pilot memory is safer that meticulous checklists. For example when US Airways Flight 1549 flown by Chesley Sullenberger ditched into the Hudson river, Jeffrey Skiles was unable to complete the checklist for sea ditching - it was too long.
There are a number of industry articles including one I wrote on the subject.
1) The Navion Flyer - Jan 2002 - What does it do? - Checklist
2) Sport Aviation - June 2011
3) Avweb (Thomas Turner) - "Checklists and Flows"
4) EAA Experimenter - 2013 - Do You Use Your Checklists?

The FAA now encourages a combination of memory and task reference. An explanation will follow.
Unless you fly a transport category airplane, or fly under an operating certificate such as FAR Part 135 charter rules, there is no FAA requirement to use a checklist, or what checklist to use.(2)
The FAA uses two primary FAR's to violate pilots that have had an accident and misused a checklist...
FAR 91.103, *“Each pilot in command shall, before beginning a flight,
              become familiar with ALL available information concerning
              that flight.”*
FAR 91.13 that forbids, *“careless and reckless operation.”*

Checklist = is a meticulous list of items to check or set and provides a specific order in which the items on the list are to be accomplished.
Worklist or Flow = conveys concepts rather then specifics. It is a reference as the pilot accomplishes a task and is dependent on the pilots memory and familiarity with the aircraft systems.
A checklist is what’s called action orientated. That is saying that it tells us each action we must take. A work list is task orientated. That is to say, it is goal orientated and looks at the big picture, and does not get bogged down in superfluous detail.
One example I gave in my article was comparing the two with a B727 opp.
"The checklist not only incorporates the normal procedures but also has the emergencies. This one list does it all. The checklist took 9 items for one of several checklists on radio operations. The work list has only 4 items and
serves as several checklists rolled into one. That is a 60% savings in space even if we don’t consider that it eliminated other checklists."
"You might be asking yourself if a work list is really this good in the
real world? Well, Captain Kunz told me that before they adopted the
WorkList for the B727, he was wading through over 500 pages in
the checklist, and as was mentioned before, a normal takeoff covered up
to 300 items."
"After they adopted work lists to the B727 cockpit, the worse case scenario on the WorkList for a takeoff was about 60 items and the total number of pages for normal op’s was around 10 pages long. That’s about an 80% reduction in the takeoff list and a 97% decrease in the total size of all normal op’s. Needless to say, in an emergency the pilots were able to use the work list more times than a checklist and the work list is much easier to remember."
Think of a work Flow as working top to bottom or left to right. When we say "check instruments", without detailing each instrument it is understood the pilot will work left to right or whatever suits his style. The B727 Checklist had about 130 items on the "before landing" list. A quick look at YouTube shows the before landing "WorkList" of a modern jet is now about 5 items!
This reduction in workload is primarily due to reliance on warning systems and the pilots memory and familiarization of the aircraft systems.


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What makes an airport "international"?
What are the requirements for an airport to be designated as an international airport in the US? Is there a minimum runway size or required customs office?

There has to be customs on site for sure.

@user3528438 You'd think, but a number of US airports with "international" in the name don't have customs facilities, as fooot points out. Sometimes the airport used to have customs and no longer does, and sometimes the airport operator just wants to sound important.

Have (or had, at one point) regularly scheduled international flights?

I live near the airport with an airport code of BLI, which stands for "Bellingham International".  It's a tiny airport near the USA/Canadian border, which varies its routes sometimes but I believe it has often only had flights to SeaTac (which is about a 90-120 minute drive away on Interstate 5) or Vancouver, British Columbia (which might actually be a bit closer).  But since there is at least one regular flight that goes to a foreign nation, the whole dinky little tiny airport was considered worthy of having "International" in its name.

Even Breda International Airport has international in it's name doesn't exaclty fly commercial flights or anything.

Cargo and GA flights clear customs at my local "regional" airport.

There is no US regulation about whether an airport can be called "international." The Secretary of the Treasury designates the official list of international airports of entry. But not all airports on this list are even called international, and it does not include all airports with international flights. There are even some airports called "international" with no customs facilities or plans to have them in the near future. The airport name is determined by the airport owner.

Note that at many US airports without on-site customs facilities, you can pay an extra fee to have officers come over from another facility for a specific flight at a specific time. Many small airports calling themselves "International" fall in this category.

It's also not uncommon for passengers on flights to the US from major airports in Canada (and a few airports in other places) to clear US customs while still in foreign territory by going through a US-staffed facility at the origin airport.
https://en.wikipedia.org/wiki/United_States_border_preclearance

Why the Secretary of the Treasury, and not like Homeland Security or something?

It is defined by the ICAO in this glossary:

International airport. Any airport designated by an ICAO Contracting State in whose territory it is situated as an airport of entry and departure for international air traffic, where the formalities such as customs, immigration, public health, agricultural quarantine and similar procedures are carried out.

"International" is supposed to be a code word to inform pilots and ATC that the Border Guards have a presence, and can admit you into the country: stamping passports, collecting duties, all that stuff.
What you shouldn't do, say, is fly from Canada into Oswego County Airport, rent a car and drive over to Syracuse Hancock International to clear Customs/Immigration.
You are to land at Hancock International, clear Immigration, then if needed fly onward to Oswego County.
The only exception I can think is if you have an emergency, such as weather unexpectedly closes in and no international airports are viable.  Aviation authorities will be sympathetic; they want you safe. The border guards will be less amused.  They will suspect a setup and that you are up to no good.  From their POV, if conditions were marginal, you shouldn't have attempted the trip.
You wouldn't want to make a habit of it.

I thought in most emergency landings, they just left you on the plane. Wasn't there a big thing a couple months ago when someone died or something because they were all left on a plane in Nunavut?

@AzorAhai Depending on the field "they" may not have the physical presence to keep a truck from meeting you and unloading a couple pallets of Kinder Surprise before they show up, or even detecting such an activity.  It's nothing new for border guards to chase an ambulance to the hospital to stamp the visa of the sick person.  Letting someone die on the tarmac is a total gongshow.

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text:
How do general aviation airports inspect their runways?
How do general aviation airports inspect their runways for cracks, ruts, holes or anything that would impair their integrity?  How often do airport operators conduct standard inspections of their runways as well as taxiways and ramp areas?

There is probably no general answer, since GA aiports run a gamut from busy fields in urban areas to grass/dirt strips in the Idaho mountains.

Runway inspection is usually performed visually. In this answer there is a list with several items which have to be checked in EGNX, but the frequency and exact list check items will be different at each airport. I'm not aware of any binding standard.
In every GA airport I know there is at least one daily inspection performed shortly after sunrise (or before the airport becomes active). This is especially important in grass airfields, where wild boars could have dug holes at night. If something happens during the day there is normally someone who sees it and can either inform the operator or fix it himself. Larger airports operating at night will probably inspect the runway more often.

But again, depends where your airport is.  E.g. Copper Basin in Idaho: http://www.google.com/url?url=http://www.airnav.com/airport/0U2&rct=j&q=&esrc=s&sa=U&ved=0ahUKEwi60fb7n93ZAhVCjlQKHW1xA9sQFggUMAA&usg=AOvVaw3_a7ekcMTe0BOXevAbEm7N with no nearby towns and seeing maybe a couple of landings per day (if that: I spent a couple of days camped there one year, and didn't see another plane.)

This video will answer your question, a reporter join one of the 4 daily runway inspections at Zurich airport. Each GA airport has its own rules about these inspections as it is the responsibility of the airport to provide a safe place for aircraft to land.

The main threat is debris on the runway which can have disastrous consequences.
https://en.wikipedia.org/wiki/Air_France_Flight_4590


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How can we design a wing for cruising at very low speeds?
I was thinking about a simple super-light aircraft powered with a little low-power piston engine. Is there any way that we can design a wing so the aircraft flies very slow (like 25 km/h as minimum speed) without stalling?
I mean a suitable wing design for this kind of aircraft. (slow one) Other questions:

Which parameters in designing wings must be considered for lowering stall speed?
My thrust power is low so I accept lower cruise speeds.whats the effects of this on my design?
Can we choose thicker airfoils for wing and ignore the drag issues due to low speed?

Basically you’re going to need a lighter weight or higher aspect ratio wings, or both (sounds like a glider, huh?).

Any wing can have any arbitrarily low stall speed - if you ask it to generate (or support) a low enough load (weight). SO the answer is related to the Power-to-weight ratio.  Look at human-powered flight aircraft for examples: https://en.wikipedia.org/wiki/Human-powered_aircraft & https://en.wikipedia.org/wiki/Power-to-weight_ratio

It sounds like you're describing a motor glider

You just need a large and light wing.

could you please enlarge effect of aspect ratio and power to weight on stall speed and generally wing design?

Why we need a large and light wing , user3528438 ?

And what about the shape of airfoil?(wing's cross section)

Wing Loading alone, without considering the power available, will not suffice. No matter how low the wing loading is, if the power is too low you cannot maintain aircraft speed above whatever the stall speed is. And although Wing Loading (Weight per unit area of wing surface) includes the weight, it omits consideration of the power available.  Wing loading (to a degree) determines the stall speed. but power available determines whether that speed is sustainable. The Power available - drag (Excess Power) must exceed 0.)

In the USAF, there is a topic which Fighter Pilots are exposed to called Energy Maneuverability, https://en.wikipedia.org/wiki/Energy%E2%80%93maneuverability_theory  which might shed some light on this for you. As an aircraft slow down, AOA increases, an the angle of the Aerodynamic force from Lift points further and further aft, increasing what is called induced drag. As an aircraft slows towards stall (maximum AOA), this progresses, until at a certain point, the slower you go, the more power is required.

If total power available is below that required to maintain flight at Maximum AOA, prior to that, you will reach a condition where specific excess power is less than zero, and the aircraft, even with full power, will continue to decelerate through stall..

I doubt that an aircraft flying at 25 km/h is useful.  Steady wind reaches 25km/h quite often (in Europe) and wind busts have even more speed. Such airplane would be hard to handle on the ground...

@TimothyTruckle: the chase vehicle for Solar Impulse was a bicycle, and the guy riding it didn't look excessively sporty. The speed the OP is looking for is not that much slower than the take-off speeds of SI, Helios and other such very lightweight, very low-power planes. Worst case, he'll be flying backwards or taking off from a standstill.

@JörgWMittag Solar Impulse was a proof of concept and never meant to be used on a regular profile neither private nor commercial. -- "Worst case, he'll be flying backwards or taking off from a standstill." of what use is an airplane that cannot return to the place it took off? Also you always have to secure the plane by fixing it to the ground while not using it to avoid unintended "unmanned" takeoff and, in result, "uncontrolled landing" aka crashing.

The first thing you need is a large wingspan $b$. This will help to spread the lift over more air and reduce induced drag, which grows inversely with the square of flight speed $v$:
$$D_i \propto  \frac{1}{v^2} \text{ and } D_i \propto \frac{1}{b^2}$$
Large wings have a high root bending moment, which requires a strong and heavy structure. Next you should add bracing, at least on the lower side of the wing. For that to be effective, you need a high fuselage so the bracing will become efficient. Yes, those wires will add drag, but at low speed this will be a lot less than the added lift-induced drag from lifting a heavier structure.

The single bracing wire can hardly be seen on this picture of the Dedalus human powered aircraft, but it is there and saves a lot of drag at low speed (picture source)
Next, pick the right airfoils. Depending on the local Reynolds number, use thin and highly cambered ones but make sure that the pressure rise on the rear upper surface is slow enough to prevent early separation. One example is the DAE31 outer wing airfoil of the Dedalus aircraft (the inner and mid-span ones are DAE11 and DAE21). At Reynolds numbers of 500,000 to one million, turbulators positioned 2-3% ahead of the separation line will trip the flow and make the airfoil tolerate steeper pressure gradients.
Pick wing chord such that the wing area gives a lift coefficient around 1, details depending on the exact Reynolds number. Lower values go with lower Reynolds numbers.

Wings that would produce sufficient lift at that low of a speed would have an enormously high induced drag. So unless you are operating a model plane that weighs almost nothing - I don't think you will have any luck.
In theory...
You would need a high cambered wing to produce lift at low speed. The wing should have a very smooth surface and max camber should be placed further back to promote laminar flow which reduces air resistance. You could also use turbulators at the seperation point. Basically anything that helps to reduce weight and drag is your friend.
To reduce the risk of a stall you should also look at slotted flaps so that a higher angle of attack is possible.
To achieve a low Induced drag you should have an elliptical wing shape and a very high aspect ratio with winglets.
Such wing could be designed without much of a problem. Your plane could however not fly without an extremely powerful motor to counter the insane induced drag. Such motor would then weigh a lot. So you would need even more lift resulting in more drag thus an even more powerful motor and so on and so forth.
Therefore just use a helicopter ;)
The slowest glider I have ever flown had a stall speed of around 60 km/h and it was an extremely light wooden aircraft. I am sure you could get a little better than that with todays technologies. But there is a reason why todays sailplanes fly faster rather than slower than they did 60 years ago.


text:
Why do landing gear tires smoke upon touchdown?
I have been observing several aircraft landings and have noticed that when the aircraft's landing gear touches down, it precipitates the rising of a cloud of smoke. Why is this?

(Source: airliners.net)

Beautiful picture indeed!

It's from the tires not the landing gear

The flight is a non-smoking one. Landing gear is the first one who get to smoke one the ground.

Because the tires are momentarily skidding on the pavement as they rapidly spin up from standstill to the jet’s touchdown speed.  It’s just a puff of burnt rubber but the tire tread itself becomes quite warm, reaching 600-700°F for an instant.  It’s also the reason a pilot does not ride the brakes upon touchdown as this could cause a blowout as the tire is ground up rapidly.

to enhance tire life, some airplanes were equipped with pre-rotator motors which were intended to spin the tires up to speed so they would not skid and smoke upon initial touchdown. does anyone know if this is still used today?

@nielsnielsen that question has been explored here: https://aviation.stackexchange.com/questions/3702/why-are-aircraft-tires-not-pre-spun-prior-to-landing-to-preserve-them

@nielsnielsen: Those systems were long abandoned, and we discussed them here

@ben thanks for the reference

Basically the same reason that car tires smoke when someone "burns rubber".

Imagine yourself driving your car at a speed of 60mph (96 kmph) and suddenly you apply the brakes completely and notice that it produces smoke. THIS IS FRICTION.
Now, when the aircraft lands at 180 knots (324 kmph) it creates friction too.

Exactly before the tyre touches the ground it is static.
As soon as it touches it takes that tiny bit of time to gain the relative speed with the airplane on the runway, but since it has already touched the rough surface. It drags on it. This is when the smoke appears.
After that it gains speed and the speed is gradually broken down by the speed brakes and it finally comes to a stop.

Except that most newer cars have anti-lock brake systems that are supposed to prevent the tires from locking up.

@jamesqf even with such systems fitted under heavy braking - stopping dead on the motorway from standard speeds for example - I've seen puffs of smoke in my rear-view mirror.

@jamesqf : They work by detecting the lock, though. That means they don't prevent it. When a lock is detected, the "anti-lock" system quickly releases the brake. This allows the wheel to rotate again. It's not 100% efficient, but from a control perspective it's much easier.

@James Trotter: I wouldn't know about that, at least from my own experience, as I try to avoid the need for heavy braking :-)

@jamesqf Unfortunately, as I have learned, it is sometimes unavoidable; When there's a collision a hundred yards in-front of your bonnet you need to come to a stop ASAP.

Anytime a tire's bottom is moving at a significantly different speed than the roadway, it'll make smoke.  An example is a burnout, where the tire is spinning at high speed with a vehicle and road going 0 mph. Roads generally go 0 mph.
Prior to aircraft touchdown, the tires are not turning, so the top and bottom of its tires are going at the same speed - aircraft ground speed, say 120 knots for a 737 in a bit of a headwind.  Not all runways have a ground speed of 0 knots, but the ones that land 737s do.
When the 120-knot tire bottom touches the 0-knot runway, that's a big difference, so as said, there'll be smoke. This reveals drag on the tire, which will very quickly spin the tire up so its bottom is going 0 knots relative to the ground.  At that point the smoke will stop.

Even the runways that do go faster than 0 knots typically don't go at the aircraft's landing speed, so there's a difference either way.

Planes land at somewhere in the ballpark of 150mph (230km/h). At the moment when the plane lands, the wheels aren't spinning, and the only thing that can make them spin is the contact with the runway. So, at the moment the plane lands, the wheels are skidding at 150mph, and they'll continue to partially skid until their rotation speed matches the speed of the plane. And it takes a little while for that to happen, because aircraft wheels are pretty heavy: according to Lufthansa, a wheel and tyre on a 737 weighs about 112kg, and about 185kg on a 747.
So, the smoke you see is material being worn off the tyres as they skid up to speed. They're designed to be used this way and are regularly inspected and replaced to ensure that not too much of the rubber is worn away.

Nice link! I would like each answer provide such further readings.

For a wheel/tire that large, those are rather light.  Contrast with a similar sized wheel/tire on any piece of construction equipment.

Sadly, your Lufthansa link no longer seems to indicate anything about tire (or tyre) weight for any sort of aircraft.

@FreeMan Thanks -- replaced with a Wayback Machine link to how it was.

It is kind of the reverse of burning tires on a car. There is a limit to static friction and tires spin. Even when they spin there is kinetic friction.  When the tires spin they get hot from friction and burn (more like the rubber vaporizes).
The tires have mass and it takes energy to spin them up the speed of the plane.  Initially the speed difference is so great they spin and smoke. The kinetic friction quickly brings the tire up to the speed of the plane.


text:
Is it illegal to partially block the aisle on a passenger airliner?
After reading all the commotion about the dog that died after its carrier was placed in the overhead bin, including the following comment:

United spokesman Charles Hobart said the flight attendant told the
dog’s owner to put the pet carrier in the overhead bin because the bag
was partly obstructing the aisle.

I wondered: is it against any FAA regulations to allow a flight to take off with a partially blocked aisle?

Offtopic but it doesn't make sense to me that the dog owner wouldn't open the overhead compartment every now and then in order to give the dog some fresh air

It is illegal.
The minimum aisle width can be found in §25.815   Width of aisle. So you could be in breach of that if you shrink the aisle too much. Similar issues are discussed here but ultimately blocking the aisle is an evacuation time issue.

More importantly it is a food and drinks delivery issue

@PlasmaHH True that. The only thing worse than an angry passenger is a hungry one


text:
How can I deal with insurance requirements for flight hours and training in order to carry passengers?
I just completed a helicopter rating and was excited to take friends for a ride in the R22 and R44. Then I learned that my flight school's insurance requires a total of 100 hours in helicopters and attendance of a 4-day course at Robinson in California to allow for taking guests. Are there alternative ways to make it legal to take friends for a flight without having to meet these requirements? E.g. can I buy insurance elsewhere?
Thanks!

It's not a legal issue, it's an insurance issue. If you take your friends on a flight you aren't breaking the law, you would probably be held liable for any incident costs though. You should simply call other places you could rent a helicopter and ask, or talk to your training provider.

The flight school might issue a waiver of that policy but it’s going to be based on a number of factors, including instructor opinion of your pilot proficiency on rotorcraft, time in the particular type you want to rent, and whether the insurance company can do this for you.  I’ve had to do that during my flight training when renting high performance and complex airplanes which had certain minimums set for their solo operation under the terms of the school’s insurance policy.  Ask to speak with the chief flight instructor for the school and see if they are flexible with a similar arrangement.  If they feel comfortable with your skill and proficiency in operating the R22 and R22 and you have at least a few hours in each type, they might just approve this.  Again, this is largely their insurance company’s call and not the flight school’s.  I’d guess the school gets discounted rates for insuring their rental aircraft by requiring pilots to undergo factory authorized training programs.


text:
Do Flight Simulation Instructors stop the simulator before a simulated crash?
There has been debate in healthcare simulation communities around whether or not to allow a simulated patient to die during training because it might negatively impact the intended training outcomes.
Do flight simulation instructors similarly stop the simulator before the simulated plane crash to avoid the student being negatively impacted (traumatized)? Is this common practice?
Update
There were many great responses to this question, to summarize, indeed simulators are stopped before the crash, but this is not related to a fear of traumatizing the student. The simulator is stopped in consideration, typically, of expensive training time (resetting motion and rebooting sim takes time).

As an aside, historically flight simulators used a camera that "flew" over a physical model. Which meant that a crash was very annoying because things could physically break...!

Mildly related, but speaking through such a case before it happens real life and giving help how to process it is a valuable exercise, I can imagine. Not sure if mixing it with routine drill is good, but then again recognizing the situations where a decision could be the last one seems like a important teaching?

That's interesting there is thought about ending a medical simulation as to avoid the trainee from being traumatized. But a patients death will eventually happen in a persons career, so why is it better to shield someone during training, knowing they will have to face this in real life? If anything, it's better that a trainee gets an idea if they can handle that kind of.... result, before they go through a lot more training/expense.

@Issel, the debate over whether to allow a simulated patient to die in a healthcare simulation arises because health care workers will encounter death on the job. The question is not whether to train or not train them on encountering death (though it is generally reported that they do not receive enough training in this area), the question is how and when they receive training regarding death. The debate is whether letting the simulator die, without structurally accounting for it as part of the training exercise, leads to an overall positive learning outcome.

Anybody traumatized by a simulator, (medical or flight...) probably ought to consider a different line of work.

@DarrelHoffman, yes, within the consumer realm, the representation of a crash is significantly banal, for both the reason you suggest (glorifying the spectacle), but also it not being the primary purpose of the sim, as well as, similar to the full commercial flight simulators, the complex physics of managing a wreck/explosion. I have heard Microsoft Flight Simulator (2020) users complain of this. Here is an example of a crash in MFS: https://youtu.be/MunbxlO0ctk

@user2705196 - Correct! My Dad showed me such a machine at Farnborough when I was about 10

@MichaelHall Not so sure. The simulation may be really good, and you can intentionally immerse so deep into the simulation that the simulation causes trauma, it will be less than the real situation of it, because you can leave at the moment that "smells like PTSD"...

@Volker Siegel, realistic as they might be, there is still the knowledge that it isn't "real".  The reactions might be entirely real, stress, elevated heart rate, sweaty palms, etc. but that isn't the same thing.  The reaction of a reasonable person following a simulator crash ought to be one of two:  1.)  Disappointment in their performance with a learning point having been reinforced, or...  2.)  "Well, now I know what an unrecoverable situation looks like".

@user253751: On the flip side, you don't want someone in a real plane that seems destined for an unrecoverable situation to get used to "oh well, looks like we'll need to restart the simulator again".

@supercat that's the same side, not the flip side

I'm not sure whether it was on here, but I did hear a little story about a student pilot asking the instructor to restart the simulator after the student screwed up, and the instructor said there were no restarts in the real world.

@user253751, yes, really... I stand by my earlier comment. This is of course my personal opinion, but it is based on years of flying and instructing in hi-fidelity full motion simulators. I flew a really crappy single engine missed approach at minimums just the other night. It was self induced, the instructor intended for me to land. I was very bummed out and hard on myself. The accelerated heart rate and sense of initial shock/disappointment was very real, but still I knew it wasn't. My point is that rational people are able to discern the difference.

"The debriefing should probably point out it's a simulator and nobody got hurt and that this is why they were training in a simulator in the first place." Expanding on my earlier comment, if this much is truly necessary to explain reality and ease trauma for an aspiring professional pilot then you may want to reconsider the IQ, emotional maturity, and overall suitability of the trainee...

I would say that crashes happen fairly frequently on initial type courses where new pilots are moving up to jets, and mainly on what are called "V1 cuts", where an engine is failed right at rotation or just after.
Many are not ready for the hard roll you get with a swept wing airplane when it yaws hard and you are slow to counteract it with rudder.  Before you know it, the wing tip is on the ground.  The instructor has to let the pilot try to recover and usually has to let it go all the way.
Otherwise, instructors will normally try to freeze the sim just before a crash because of time wasted waiting for the machine to reboot and having to reconfigure all the settings the instructor has set up for the session. This wastes very valuable and expensive sim time (I was also told it was hard on the equipment, but I'm not sure why).
It's not done to avoid traumatizing the students though.  One basic tenet of training is to make things memorable, so overall, a crash-in-the-box is a plus.
You can get a bit lost in the moment and forget you are in a simulator if the pressure is dialed up high enough (after 4 hours of abnormals and emergencies, you get wound up pretty tight; I used to tell people it was like spending hours in a cement mixer filled with bowling balls), but that effect is pretty superficial and when a crash happens, you know you are on an amusement park ride in the end, and you're not going to wet your pants in your last simulated moment of life.
In fact, aside from the visuals, which on the latest ones are pretty good but still not like a real view, after you get used to it, you start to become more and more aware of the physical differences from a real flight, like the motion sensations that don't quite fit what you are seeing (they really only simulate accelerations and decelerations and short term jerky up/down and rolling/yawing movements with any realism, which is good enough most of the time), and the sound of the hydraulic gear working below you, and that tends to kill the "lost in it" effect even more, and it becomes more and more like an amusement park ride as you go back for recurrent training.

The "hard on the equipment" is likely because of the motion system. The mathematical model is normally not meant to simulate a crash, or even an impact properly. If the software doesn't abort automatically, its behaviour (in terms of simulation) may be basically unpredictable. This may result in large abrupt demands for motion, which can be stressful for the equipment as well as for the occupants.

@Zeus I would assume that the software is set up to limit loads to those that won't significantly damage either the simulator or the occupants thereof. A hard landing sure can get the attention of everyone standing around in the room where the sim is, though. :) You can't see what's going on from the outside, so you go from just seeing very light, gentle motions to the thing suddenly slamming against the stops. - haha - It feels like a pretty big bump from the inside, too, but not enough to injure anyone. Not that I've ever had a hard landing in a sim or anything...

@reirab, Not "significantly damage", of course, but "hard on equipment" it can be. Like 101% N1 will not damage the engine, but will shorten its life... Yet artificially limiting actuators to very safe motion would prevent simulating certain effects in the infrequent cases when they are actually helpful. Often, things are more complicated than just a poweful stroke: for example, motion system can induce vibrations that are not healthy for the cabin, the equipment or people.

instructors will normally try to freeze the sim just before a crash because of time wasted waiting for the machine to reboot and having to reconfigure all the settings - If it is such a big problem, why is there no automatical system to do so? I would think it should be trivial to restore a saved previous state of simulation, regardress of the current state. Why would whether the plane crashed or not affect this?

How old are these simulators that take a while to reboot?  Presumably airliner models that are nearing retirement still have operational simulators; they might be 25 years old or so.

@Neinstein In a pure-software simulator, that would indeed be easy. The moment you start combining it with hardware, both controls and actuators, it gets a lot trickier.

@Mast It shouldn't be if the controls/actuators have an absolute location/state monitoring system, which I would assume to be the case in any moderately advanced skinetic simulator system...

"You can get a bit lost in the moment and forget you are in a simulator if the pressure is dialed up high enough" --- Does the opposite ever happen where a pilot thinks that he is in a simulator when he is in a real plane?

@DrMcCleod I doubt it.  When you are in the real plane it's obviously a whole new world.

@Neinstein, not necessarily. Many don't have absolute position sensors (I know for a fact). But this is not even a big problem. Modern simulators are a multi-computer (and multi-unit in general) system. If you restart it, it may need restart, reinitialisation and self-test of all sub-systems, etc. Sometimes (intentionally or not) the avionics will lose the settings/flight plan/etc, and it will need to be set up again. In many cases restart could be optimised to be faster, but as usual in aviation, it is not practiced if it has even 1% chance of incomplete or inconsistent recovery.

In the rare cases in the Shuttle Mission Simulator when a training session entered a loss-of-control or other irremediable situation, it was standard practice to freeze the sim rather than run the case to the point that it auto-froze due to ground contact or excessive acceleration.
Once it was clear that the situation was irrecoverable, it was a waste of expensive simulator and personnel time to continue.

In my part 91 (VFR, general aviation) training my CFI was more than happy to let me crash in the sim. Others have spoken about sims that took time to reboot/reset, but in the case of the single-engine simulators I used it's all managed via tablet more or less instantly. I definitely "tested the terrain system" in the sim a few times without it being treated gravely.
To directly answer your question - no, in my experience there was no special treatment about the sim. It was understood that it wasn't real, that this was training. If i'd done that in a real plane I (and any passengers) would have died, but that's the point of the sim. The flight school did the same for part 141 and part 91 pilots, so presumably those on their way to the airlines would go through the same experience.
However, given the subtext of your question (training medical workers), it's not quite the same real-life situation between aviation and healthcare.
In an aviation incident, pilots and passengers are both similarly likely to be injured or killed from accidents. Crashes are not expected or common, and if it happens you'll have more than psychological trauma to worry about.
Whereas in a medical situation you're expected to see death routinely. A nurse or doctor might see such a thing on as much as a daily basis, through absolutely no fault of their own or anyone else's.

As you say, if you crash a plane in real life, you probably die yourself.  But in a sim, you know, on a deep internal level, that didn't actually happen to you because of the fact that you aren't dead.  Losing other (simulated) people you were trying to save may be different, because you don't have that gut-level "that didn't really happen" feeling.  (I haven't done either medical or aviation training so this is just guesswork on my part).

Training to remote-pilot a hypothetical crewless passenger plane could be the middle ground here: you're still thinking of aviation factors, not directly about the human lives you're trying to save as it slips away from you, but what you experience would be exactly like if you'd lost a real plane full of pax.  If real planes losing real pax this way was a real thing that you were worried about, I could imagine that being a bit more potentially traumatic.


text:
Have there been accidents involving flight simulators?
Have there been any accidents which have happened to/in flight simulators?
I understand that simulators are used to simulate general accidents for training purposes (engine failure, birdstrikes etc), and also specific accidents for forensic purposes (the first use, in 1972, of a CAE JAT DC9 SIM), but I am wondering if there have been any accidents with the simulator itself? I am thinking of possibly a simulator's motion system failure? or fire? or trainees trapped? etc.
Additionally, if you knew of any source which might keep record of this type of information, that would also be very helpful.
The example that first comes to mind is the tragic 2014 crash of a Beechcraft King Air B200 into a flight simulator in Wichita, Kansas.

Update: To clarify, I am interested in all examples, but those that have received media attention for whatever reason are my primary interest (these stories will be held in the public record and can thus be further researched). Though interested in “work-place incidents”, these would likely have been dealt with internally unless there was a certain severity or sensational aspect.
I have answered my own question for now, will reward answer to someone else if they have any further information.

Yes there has been at least one, where the simulator tips over, but I can't seem to find it.

Whenever I was in the sim I would imagine it getting pissed off with its job and ripping up its anchors, smashing through the wall and running off down the street.

How severe does it have to be? That 2014 crash is famous, and probably the only fatal one. If you count non-fatal incidents, there must have been many, and they would be counted as "workplace incidents". I know of at least one with "trainees trapped" due to door failure, but in the end that just amounted to wasted time. (Had there been a fire at the same time, it could be fatal). Also, there must have been many incidents, including injuries, while building simulators.

Not strictly aviation. I once read of a railway simulator in the UK in the 1960s designed to speed up the transition of steam drivers to electric locomotives. It had a replica cab on hydraulic actuators, projected movie film of the track in front, and controls like the new locos. Film speed,  cab movement, etc, controlled by a computer. It mimicked the sensations a driver would get when accelerating, braking, rounding curves, etc. One person drove into the buffers at the end of the track and the simulator was so faithful he was thrown forward and got a broken nose.

One of our STS-86 training team members fell down the ladder in the Shuttle Mission Simulator fixed base and broke his leg. Rhea Seddon famously broke her foot during escape training in the Full Fuselage Trainer (that was not really a flight simulator, more of a mockup).

@RonBeyer, was the simulator tip over mentioned in the answer below?

That's one yes but the one I was thinking of originally was, upon further research, aviation satire.

I would be curious to read it, @RonBeyer, if you wouldn't mind sharing? Given the types of results I have come up with in my own research, satire has a place (at least in the comments) of this thread.

https://aviationhumor.net/student-pilot-crashes-simulator/

I have been attempting to find information on this question for quite a while, and judging by the lack of answers, I am assuming that user Zeus, in the comments to the question, is right: that accidents involving a flight simulator, to the extent that they are severe or sensational, are very rare. More often, accidents would be considered “workplace incidents” and would go unreported outside the company.
In Larry Milberry’s text The Story of CAE (2015), while discussing safety procedures and accidents in the development of flight simulation motion systems, several accidents did indeed happen, but none which involved injury:

There was a failure of a British Airways simulator where “a
malfunction drove the cockpit at maximum pitch-up velocity into the
end of travel, breaking the upper bearings and sending the simulator
to the floor.”
During the testing of the (never installed) Iranian CH-47 simulator
“there was a motion system failure. The simulators unusual vertical
configuration meant that any uncontrolled loss of hydraulic system
pressure would cause the flight compartment to lurch nose-down…
The potential for Murphy’s law to take over was there in spades, and
one day the interlocks did fail…”

In my pursuit of this question, working through historic newspapers, I have come across several tangentially related, though ultimately unsatisfactory, examples to my question: the 1967 oxygen fire in the Apollo 1 capsule simulator that killed 2 airmen (and 16 rabbits), an eerie repeat of the explosion which killed the 3 astronauts the Friday before; the 1991 B52 crash during the Gulf War which again was an "eerie repeat" of a “crash-in-the-box” during previous training; the 1991 deaths of two pilots “simulating” the attack on Pearl Harbor in Florida; the 2001 attempt by a man to open the emergency door mid-flight saying “it’s OK, we are just on a simulator”; and finally, many instances where flight simulators have been used to familiarize those with intent to hijack a plane.
Again, these are not what I am looking for, but they are the types of things you find when you start searching for this particular information. My sense is that there must be examples of accidents in flight simulators which have been significant but which, likely due to them being workplace accidents, did not receive media attention. Nonetheless, maybe this answer will help provoke the memory of others to contribute a more precise answer.

There was a cherrypicker positioned to retrieve crewmembers from the Shuttle Mission Simulator motion base if it ever got stuck (arrow). Never used.  https://i.imgur.com/vQozV8Q.jpg

In recent history, an 'evacuation trainer' caught fire in 2016
In my experience wrist injuries, broken arms, etc. on FFSs can sometime happen, especially if the instructor does not wear the seat belt - 'glitches' in the visual database while taxying on the ground can sometime result in sudden accelerations.

The 2014 incident is listed in the question.

Sorry i must have missed it. I edited the answer


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What causes the radar phenomenon of ‘far-shore brightening’?
In Larry Milberry’s text on CAE (Canadian Aviation Electronics), in a discussion of the development of digital radar land mass simulation (DRLMS) for the Dutch Lynx mission simulator, far-shore brightening is described as “coastlines [appearing] brighter, because there are two different radar returns—one directly from the radar scanning the coast, the other caused by the radar reflecting off the water onto the shore.”(p.240)
Now I know this would seem to answer my question, but I have had difficulty finding any other definitions of the phenomenon.
Specifically, I am wondering if there are other important details about far-shore brightening that are being missed?

Is far-shore brightening specific to aircraft radar due to the high
angle relative to the ground? Or is far-shore brightening also a marine radar and land-based radar phenomenon?
Is far-shore brightening common on contemporary radar? Or has there
been advancements to compensate?
Is far-shore brightening helpful when reading the radar?

If you might be able to direct me to further resources that would be very helpful.

A picture of a radar display showing far-shore brightening would help the question.  I did a quick web search but didn't have any luck.

Can you give the name of the book please? I worked on the B-52 simulator which also had a DRLMS, and I'd love to read this book.

@OrganicMarble, I will add the link into the post. The text is a sweeping history of CAE, which contains little specifically on DRLMS, but you will no doubt find much in it (particularly the many anecdotes) interesting! :)

Short answer
Double-bounce backscatter or far shore brightening are radar industry phrases referring to the physical effect of multipath propagation: A target is illuminated by a direct ray and also by reflected/indirect rays. All rays are scattered back to the antenna. Indirect illumination can happen after reflection on the ground or on water, particularly salted water which is more reflective than fresh water. A typical case is the double bounce on wetland ground and vertical tree trunks.

Radar double-bounce backscatter
Calm water and flat ground make good mirrors for microwaves, and when not orthogonal to incident rays they reflect them away from the radar antenna. the related areas appear black on the screen. This contrasts with bright double-bounces.
The same effect happens in urban areas with bounces on ground and buildings:

Double-bounce backscatter in urban area, source
On the picture above, the radar beam comes from the right/east side (as indicated by the green arrow). There is a street crossing NW to SE which appears in black because this is a smooth surface at the wavelength scale, with mostly specular reflection. However reflected rays are at the optimal angle to reach the buildings on the west side of the street and return to the sensor. The buildings are illuminated both by the direct rays and by the rays bounced by the street material, some of these rays are scattered back to the antenna, the radar cross-section (RCS) of these buildings is overrepresented. None of the buildings on the east side appear as bright, because the double-bounce is not possible.
In multipath propagation, the resulting power depends on the phase difference between the received rays. The general case where a lot of rays are involved (diffuse reflection) leads to phase differences randomly distributed and no higher reflected power, but the case when there are only two rays involved (two-ray ground-reflection model) can lead to over brightening. This happens when the phase is the same, that is when all reflections occur on surfaces tangent to the first Fresnel volume boundary, for reasons explained in the additional sections.
Two-ray reflection is frequent in satellite imaging (dihedral or trihedral return). There are a few techniques to mitigate it, e.g. based on transmitter and receiver antenna with different polarizations. But these anomalies are actually used to extract more information from the radar image, this is the ground for radar polarimetry which exploits the fact that each time a bounce occurs, the signal phase is reversed. Polarimetry uses extensively DSP to split echos according to their type, and create synthetic representations that transform initial over-brightness into something else, thus eliminating over-brightness (more).
Sum of two waves
When a reflection occurs, the sign of the signal amplitude is changed, this corresponds to a phase shift of 180°. The target is illuminated by two waves with opposed amplitudes, but one has traveled a longer distance, hence it has been also phase-shifted by an additional amount (180° + x°). Interferences can be constructive when amplitudes have the same sign, or destructive when amplitudes have not the same sign. Let's imagine 20% of the wave is reflected (orange below) and let's compute the sum for various delay (phase) values:

Direct and indirect waves interference
When the additional phase shift is between -90° (+270°) and +90 the interference is constructive: The sum is larger than the direct signal (the actual range varies depending on the ratio between direct and indirect waves amplitude).
Volume of constructive interferences
This range corresponds to a range of additional travel time for the indirect wave, and to a range of distances from the centerline (line of sight) for the location of the reflection.
These points of reflection are within a volume bounded by an ellipse having the radar and the target as focuses. It's named the first Fresnel ellipsoid, after the great engineer and physicist.

Multipath propagation within first Fresnel ellipsoid
For 2GHz and a distance of 30 km between radar and target, this volume is very thin. The largest diameter of the ellipsoid, found at equal distance from focuses, is only 68m (calculator). For an airborne radar, it means the reflection we are interested in can only occur close to the shore. There are other larger Fresnel ellipsoids confocal to the first one, and half of them allow constructive interferences, but as they are more off-center, the energy drops rapidly, in practical only the first ellipsoid is taken into account.
Mitigation
Multipath effect is often mitigated by diversity, which means using multiple frequencies, or multiple antennas.

An example is GNSS (GPS, Galileo, ...) where frequencies are diffracted by ionosphere, introducing a pseudorange error. Different frequencies are diffracted differently. For authorized users, the diffraction error can be quantified, and corrected by measuring at different frequencies. Similarly because reflection depends on Fresnel ellipsoids, which size is different for different frequencies, multipath can be quantified in radar.

The antenna diversity can be based on sophisticated technologies, the phased array antenna is a most simple one. The array is composed of tens or hundreds of small antennas. The signal they transmit or receive can be delayed individually. This allows to sense multiple wavefront planes at the same time, and identify reflected wavefronts.

MIMO approach (multiple inputs, multiple outputs) is based on both diversities. MIMO is also used for 5G cellular networks to take advantage of multipath propagation (if your WiFi access point has several external antennas, you're likely using MIMO too).
As said earlier when a wave is reflected its phase is changed, but its polarization is also affected (polarization is the plane the electric field of the EM wave lies in). Echos can be identified as resulting from an odd or even number of bounces, thus double-bounced rays can be removed.
Another technique consists in preventing waves from leaving the reflecting material. Reflection must be transformed into a refraction. Waves can be polarized in a direction parallel to the plane (p-waves) or perpendicularly to the plane (s-waves). At some angles, close to Brewster's angle, only p-waves can leave the reflecting surface, s-waves are refracted and absorbed by the material. Reflection can be eliminated this way too, but this requires a specific location for the radar and the capability to create s-waves.

Is far-shore brightening helpful when reading the radar?

Are you needing the radar to see the shore? If so, it's helpful. Otherwise it's a distraction or just a non-issue.

Is far-shore brightening common on contemporary radar? Or has there been advancements to compensate?

Does it need to be compensated? Is it a problem? Generally speaking, the only way to compensate without using passive or synthetic aperture radar is to use multiple wavelengths. Broadband radars are not typical in civilian applications, but certainly are widely used in military applications, where they almost entirely sidestep the effects of destructive interference. Such systems are very expensive due to more complex RF paths.

Is far-shore brightening specific to aircraft radar due to the high angle relative to the ground? Or is far-shore brightening also a marine radar and land-based radar phenomenon?

It can happen anytime there's multipath propagation via paths that have similar length (within a 1/4 wavelength of each other + a small integer multiple of whole wavelengths), so it's not only not specific to aircraft radar, and it's not even specific to shores.
On a hot day, using an amateur K-band radar mounted on a tripod, you'll get "far shore brightening" of returns from cars further down the road. The hot air layer that hugs the road and causes optical illusions does the same to radar that has wavelength shorter than the thickness of the hot air layer.
If the multipath propagation has paths that differ by enough wavelengths, the target is broadened in radial direction (away from the radar), up to discrete echoes being recognizable. This usually makes the target easier to see, since there's more of it, but it's also harder to discern details obscured by such an enlarged target. It all depends on what you need from your radar image. It is rather benign in peacetime applications.

You can do passive radar experiments at home without anything special :) One can hear multipath propagation with a ceiling fan acting as a variable reflector/attenuator target, a broadcast UHF station as the radiation source, and a slightly off-tune FM radio as the receiver. It's easy to hear the frequency of the blades, and if you put a large sheet of metal nearby, you can hear the effects of interference from multipath propagation. Constructive interference makes the signal much easier to hear!


text:
Which countries in Europe allow for the use of Pilot-Controlled-Light?
In Europe is very uncommon to see this feature at airports due to the local regulations in different countries, but still some are allowed and use this system. What countries allow for its use? Please name at least one airport for each country which uses it.

Are you really asking us to list every single airport in every country in Europe which uses this? That seems exceptionally broad.

@Ghilardi, unless you write a perfect question (hint: nobody does) there will always be some people that will have doubts on how to answer it, please accept it as a demonstration of the interest those people have in your questions.

Sweden has numerous airports that use pilot controlled runway lights. Such lights are typically turned on by making a long (10 seconds or longer) transmission on the airport frequency, though variations exist.
Some examples of airports in Sweden that use PCL are:

ESSD Borlänge/Dala Airport
ESGE Borås
ESSU Eskilstuna/Kjula
ESNG Gällivare
ESSG Ludvika
ESKM Mora/Siljan
ESND Sveg
ESGT Trollhättan/Vänersborg
ESKC Uppsala/Sundbro

For clarity's sake: This answer is not meant to imply that other countries do not allow for PCL, nor that all lighting on the airports listed is pilot controlled.


text:
What other circumstances may cause the autopilot to trip out?
I recently watched Air Crash Investigation, Air France Flight 447.
The autopilot handed back control to the flight deck after the pitot tubes temporarily froze. The autopilot knew it was getting naff information and said: "We can't deal with faulty information so we are handing control back to the flight crew".
This got me thinking. What other instances, beside instrument error, if any, will the autopilot trip out, and hand control back to the crew?
An initial Google search was not helpful I am afraid. A pdf of the Airbus systems (350 pages long) was available, and a news source on everything the autopilot does, but nothing mentioning examples where all of a sudden the autopilot may switch off, even temporarily.
The reason I ask this is that if this is the only reason the autopilot may trip out, surely the crew of Flight 447 would have been aware that it was due to information that was not correct, and therefore to just remain as they were until the sensors came back to life again.
That is what I can not understand.

There are many such situations. Any erroneous flight data, failing self-test, any parameter going out of its allowed range, and of course several possible flight crew actions. A full list might be too much for an answer.

@DanHulme will it not inform you of why it has tripped out?

Part of the Air France 447 crash wasn't just that the autopilot disengaged, but that it switched to alternate law. Normally, the A330 won't let you put it into a stall, but alternate law will. It's assumed that this is why the crew ignored all the stall warnings.

What other circumstances may cause the autopilot to trip out?

There are numerous circumstances, a few at random

pilot presses the autopilot disconnect pushbutton.
applying force to control-stick that contradicts autopilot.
entering a stall.
activation of high AoA protection.
descending below MDA on a non-ILS approach.
inconsistent data.

will it not inform you of why it has tripped out?

Pilots check the ECAM for messages and suggested actions.
The AF447 accident report contains ECAM messages, note the ones at 02:12:44:

The Pilot in charge should assign which pilot handles ECAM messages, they should then have applied the ADR Check Procedure

1.16.8.2 Case of speed display anomalies
The philosophy for processing the anomaly is described in the “UNRELIABLE AIRSPEED INDICATION / ADR CHECK PROCEDURE” that is in Flight Manual (AFM) and transcribed in the Airbus FCOM and the Air France Operations Manual.
Airbus
The FCOM procedure indicates that:

The crew identifies the loss of consistency in indicated airspeeds;
And if the safety of the flight is affected by the indicated speed anomaly, and until the aeroplane reaches the safety altitude or the aerodrome circuit altitude, the crew first performs the  Memory items indicated in the “UNRELIABLE AIRSPEED INDICATION / ADR CHECK PROCEDURE” inset. The objective of these memory items is to maintain the aeroplane within a safe flight envelope and to stabilize the flight path to allow time to find, in the QRH, the tables giving the more precise pitch attitude and thrust values to be used for the flight:

Notice that the first two items are for the pilots to make sure the autopilot (AP) and autothrottle (A/THR) are OFF.
The pilots did not systematically work through ECAM messages and did not apply the ADR check procedure (one they would have memorised in training). They were confused and acted erratically. They stalled the aircraft, ignored the stall warnings and held the aircraft in the stall from above it's operational ceiling right down to sea level.
The sad truth is that the only action really necessary was to keep the wings roughly level for a few minutes. It is possible that if both pilots had folded their arms, tucked their legs back and taken a 5-minute nap, everyone on that plane would still be alive.

Glossary
- ADR = Air Data Reference
- AFM = Flight Manual
- ALT = Altitude
- AOA = Angle of Attack
- AP/FD = Autopilot / Flight Director
- A/THR = Autothrottle
- CLB = Climb (throttle setting)
- ECAM = Electronic Centralised Aircraft Monitor
- FCOM = Flight Crew Operating Manual
- ILS = Instrument Landing System
- L/G = Landing Gear
- MDA = Minimum Descent Altitude
- MSA = Minimum Safe Altitude
- NAV = Navigation
- PF = Pilot flying
- PIC = Pilot in command
- PROC = Procedure
- RED = Reduction (thrust reduction altitude - during climb after take off)
- SPD = Airspeed indicators
- TOGA = Take-Off / Go-Around (throttle setting)
- QRH = Quick Reference Handbook

Many thanks for this answer. It is helpful. Would you mind explaining the error messages you have highlighted?

@cmp I suspect AIR SPD X CHECK is "air speed cross-check" (check whether the various air speed instruments show the same value). Rather surprising the computers can't perform this check automatically, though. AOA is almost certainly Angle of Attack. I don't know off hand what ADR would be.

Pardon me for being a bit thick, did these error messages simply not help the flight crew of AF447?

@cmp: The AF447 pilots failed to coordinate their actions, allocate work, communicate effectively or to work systematically through the ECAM messages. The pilot flying pulled the control stick hard back and kept it there for over four minutes from initial upset to impact, ignoring the stall warnings completely and releasing pressure only for a few seconds. We do not know what was going through his mind. It seems he was completely confused and at a loss and his physical actions are inexplicable and completely counter to what is expected, he disregarded altitude and warnings.

You give a good answer. Thanks.

There could be lot of contributing factors:

Environmental Factors: Freezing conditions causes pitot tubes to block, painting in and around pitot tubes without protecting pitot tubes
Insects in pitot tubes or any blockage
Technical Malfunction: Hampered Power supply to autopilot or any instruments operating
Human errors: A sudden Elbow press to turn Autopilot off

But in all these circumstances, alarm rings and alerts are shown. These are known conditions from various air crash Investigations.


text:
Why would lift coefficent change with weight on swept-wing aircraft?
Today I encountered a problem. There is a book saying, "Why does the lift coefficient of a swept-wing airplane decrease with the increase in the weight of the airplane?" I cannot think of the reason. Isn't the lift only coefficient related to the state of the wing?

Welcome to Av.se! You might consider using a more descriptive title for your question; as is, reading the title doesn’t really indicate what specifically your question is about.

A swept wing has less torsional strength than a straight wing, ie there's less structure at 90d to the root supporting the tip of a swept wing.
Thus a swept wing twists when it's loaded, which unloads the tips and thus generates less lift at higher weights.

Unless it's swept forward!   https://www.bing.com/images/search?q=x-29+aircraft&FORM=IARRTH&ufn=grumman+x-29&stid=68773e7e-20bc-546e-c7c8-f7a15cb602a4&cbn=EntityAnswer&cbi=0&FORM=IARRTH

No @Charles Bretana forward swept wings have very different torsional and stress-strain characteristics and because different sweep angles are optimized for different Mach numbers, the lift generated is not really proportional or comparable to back swept wings.

Who said it was? All I am saying is that it does not twist in a manner which "unloads the wing tips and thus generates less lift at higher weights". In fact, unless constructed to eliminate twist due to the airloads, it does exactly the opposite. But it's a moot point because forward swept wings are the exception, not the general rule.


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What is the difference between RAIL and SFL lights?
What is the difference between Runway Alignment Indicator Lights (RAIL) and Sequenced Flashing Lights (SFL)?
Both of them are same in terms of flashing light.

From Wikipedia: "RAIL are similar to sequenced flashing lights, except that they end where the white approach light bars begin."

SFL start at the decision bar. RAILs may not.  The key differentiating factor is that RAILs extend beyond the centerline lights.

Thank you for answer. In the other words,SFL is embedded in ALS, on the  other hand RAIL is outside of ALS. Is it right?

@Aviat Lol! "Hand rail" had me confused for a second. [forehead slap]

There are RAILs and REILs. Had a instrument student get dinged on that once.


text:
Why is it necessary to fly in visual conditions to certain unmanned airstrips?
I have come across certain unmanned airstrips where only VFR flight is allowed not IFR? I wonder what can be the possible reasons for that?

Can you give us an example of an airstrip, and tell us exactly what it means that IFR isn't allowed? Where did you read that? Also, are you asking about regulations in a specific country, or is this a general question?

Two reasons:
1) It takes time and money to create an instrument approach procedure, and possibly not enough people have requested that one has been created.
2) A certain amount of obstacle clearance is required, if for some reason the required safety cannot be guaranteed using instruments, the FAA can simply not create a IFR approach

I don't ususally like changing the words of others, but I made a minor edit because based on the way the question was asked I couldn't be confident that the OP would understand what it was you were refering to in #1 above.

I’d have to examine the specific airport, but you are required to have an approved instrument approach procedure, with current weather minimums above the published minimums, in order to approach and land at an airport when the weather is below VFR minimums for the airspace the airport is in.
Some airports also have terrain and other nearby obstacles which may make it hazardous to takeoff and land from in marginal weather or at night.  Check the Chart Supplement for that region for further details.
This does not prohibit flying out to that airport under IFR, though.  But the local weather at the airport must facilitate the ability to descend from the MEA and make an approach and landing under basic VFR.  Ref §91.169 (c)(2)

Instrument approach relies on beacons and transmitters on the ground like VOR/DME or ILS.  That's unlikely to be in place.  Furthermore, even if this was in place, a manned tower would be needed for ground movements in IMC conditions, to avoid runway incursions.

At least in the US, plenty of instrument approaches are GPS-only, with no ground transmitters at all. And many non-towered airports have instrument approaches.


text:
What is the origin/history/background for performance categories A,B,C?
What is the thought process behind performance categories A,B and C?
I know they were done based on MTOW 5700 kg, 9 pax and engine drive.
I understand the table of classification, but I would like to know the thought process behind that. No ATPL book has explained it in a good way.
Thanks in advance.

I believe that some apparently random breakpoints were created due to specs and capability of aircraft in common use at the time.

I would like to know the background and why it is the way it is. Thanks.

Understood, if I knew more details I would answer.  Sorry!  I’ve read somewhere, (maybe here) that the weight or passenger count of the popular DC3 formed the basis of a category break decision.  Have you tried searching here or elsewhere?  Maybe my comment will prove a clue to a keyword search that gets you to an answer…

See this question and answers, perhaps it answers yours

Please include the jurisdiction you are asking about. ABCD[E] performance categories are used entirely different in the USA and designate the design speed ranges for instrument approach minima, totally independent of aircraft mass.

@MaxPower...jurisdictions is JAR. But it's not about the speed. It's the performance categories.


text:
What is the physics behind if for conventional VOR, if the transmitting antenna rotated, amplitude modulation is produced?
Why for conventional VOR, the transmitting antenna is rotated? And how does it produce amplitude modulation? What is the physics behind this?

This question is similar to: What is the difference between conventional VOR and Doppler VOR?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how this answer on that question was not helpful for your problem.

Also the wording of this question sounds a bit homework-y, which is mostly OK but you have to put a bit more effort in if you want help.

The antenna could rotate, but usually it doesn't, only the field rotates. From the linked question: The cardioid EM field which constantly increases and decreases in strength at a given point in space as the pattern rotates is sensed by the receiver as a constant signal modulated in amplitude at 30 Hz, this amplitude variation represents the variable signal. The phase shift between this signal and the reference allows to determine the bearing.

@mins sorry I don't understand. I read your entire answer but still didn't understand. Can you direct me to the basic physics behind this?

@mins how amplitude modulation is produced with phased array?

@mins what is the physics behind this? Any keyword you can offer me?

Answering in chat.


text:
Why are the speed brakes on the F-86 deployed when parked?
The lovely F-86 Sabre always seems to have its speedbrakes deployed when parked, to the point that most of the results I found looking for a good photo for this question were scale model forums debating the exact angle at which to position them.

(Source)
However, none seem to offer more than wild speculation about why they drop. The guesses mainly fall either into "it's held up by hydraulic pressure, so they fall down when the hydraulics are off" or "it doubles as a maintenance access door for some kind of ground servicing"... but with no evidence either way.
Interestingly, the FJ-2/FJ-3 Fury carrier derivative seems to not have had this quirk, even though my limited knowledge is that the changes were mostly just in the wing and landing gear.

(Source)
It also looks like the F-86D Sabre Dog and its descendants parked with the speedbrakes stowed, although that was a different fuselage design.
So my two-part question is:

Did the speed brakes dropping open when the F-86 was parked serve some intentional function (such as servicing or inspection access), or was it just that they didn't have a mechanical lock to hold them up?
Which of the many variants/derivatives of the Sabre had the droopy speedbrakes?
For bonus points, what was changed to keep the speedbrakes up on variants like the Fury, and why?

The F-86 Sabre Pilot's Flight Operating Manual is availlable on Google books site.
It describes the speed brake system as being operated by the utility hydraulic system. Both extension and retraction are hydraulic, and the operating switch also has a "neutral" position, which locks the speed brake into its current position by inhibiting hydraulics flow. The neutral position is to be used whenever the desired position (open or closed) is reached to minimize fire hazard, as the hydraulic lines are placed close to the engine. The speed brakes do not have mechanical locks. Emergency closing is provided by a swich which depressurizes the actuators allowing airflow to close the speed brakes.
The reason for the speed brakes being open while parked is simply that the manual states in the "Stopping Engine" checklist item 5: "Speed brake switch - out". The manual does not elaborate on why the speed brakes are selected out for parking.
The carrier derivate obviously has different checklist omitting the speed brake-out item.
The speed brakes being open while parked does give the pilot (and mechanics) the possibility to externally inspect the system (hinges and such) before flight, and verify the operation (closing) without fully cycling the system during preflight check.

Preflight check might also include things like "Check for bird's nests inside speed brakes".  Seems like it'd be a tempting nook for some feathered creatures.  I have to assume that's happened at least a few times, especially if a plane has been sitting around for a while...

Love the illustrations in that manual! It does mention "dump utility hydraulic pressure by operating speed brakes" after that checklist item 5 - I would imagine that dumping pressure "safes" the system (for servicing, and reducing chance of hydraulic leaks near the hot engine), and the speed brakes are just a convenient way to do that. I'll leave this open a little longer in case someone wants to chime in why the Fury had a different procedure for the fake bonus points. :)

The manual or procedure or authorities or law or whatever states so is never a REASON. That is the QUESTION: why the manual states what it states.

Are you seriously questioning the causality between the PFOM and the air brakes being open while parked? Why PFOM commands air brakes out is a valid question in itself, I'll give you that.

@Jpe61 I don't think the question is really asking "why do the pilots open the speed brakes" (to which the answer is obviously "it's in the checklist"). The question is interested in the purpose of opening the speed brakes: i.e. "Did the speed brakes dropping open when the F-86 was parked serve some intentional function". That the checklist says to open the brakes is evidence that there is a purpose (or why was that step written into the checklist), but leaves unanswered what that purpose is. This question is the "Why PFOM commands air brakes out" one you acknowledge as valid.

The headline: Why are the speed brakes on the F-86 deployed when parked? and the 1st part of the actual question in full: Did the speed brakes dropping open when the F-86 was parked serve some intentional function (such as servicing or inspection access), or was it just that they didn't have a mechanical lock to hold them up?. Neither  mention PFOM as the PFOM item was not known to the OP. My answer gives a direct reason for the speed brakes being out, removing speculation about possible mechanical locks and lack of hydraulic pressure which both are speculated in the question.

I think @Jpe61 has answered the gist of the question - why are they always down on parked planes? "Because procedure is to open them when parking to depressurize the utility hydraulics, not because they fall down on their own." I've only put off accepting it as an answer in case someone comes along with more about why that procedure apparently changed for the Furies and Sabre Dogs. Interestingly I found a manual for the Fury that has the same system description but doesn't include the checklist step to open the speed brakes. https://www.vmfa251.org/planes_fury.htm

It's a military plane, when they tell you speed brake - out, you set speed brake out. You don't ask why

@Jpe61 while that might be good advice for pilots it's poor advice for aviation engineers. Someone has to write these things.